Macrophages are involved in a variety of processes, many of which affect the eye. The present studies have dealt with three macrophage interactions: (1) Macrophages produce and release a mediator, lymphocyte activating factor (or Interleukin 1), which is essential for stimulation of the immune system. The relationship between production of Interleukin 1 and the degree of differentiation of human monocytes to fully mature macrophages was examined. (2) Macrophages release also a mediator which stimulates fibroblasts and is believed to participate in processes of fibrosis and wound healing. The possible identity between this mediator and Interleukin 1 is suggested by data accumulated in this study, mainly from experiments using silica particles for macrophage stimulation. (3) Macrophages are a major component of the inflammatory infiltration and are known to adversely affect adjacent tissues. The possibility that macrophages may participate in cataract formation, a common feature of chronic uveitis, was examined by determining the cytotoxic effects of inflammatory macrophages on the metabolism of lens epithelial cell cultures.